Concrete ties containing prestressed wires are in common use, but in order for them to be economically viable it is necessary for them to be produced at a very fast rate, and the most usual practice of producing concrete ties is to stretch out prestressing wires over a considerable length, the wires being contained in a series of aligned cavities of multi-cavity moulds, the concrete being poured into the moulds and allowed to set while the tension is retained on the rods. After the concrete has set the tension is released from the ends of the rods and the rods are cut between the moulds and the ties are removed from the moulds.
Many proposals have been made for the production of concrete ties (sleepers) and other concrete articles. For example Barber in his U.S. Pat. Nos. 2,394,227 and 2,394,228 teaches formation of prestressed concrete sleepers where moulds may be aligned both end to end and side by side. Barber is primarily concerned with a single cavity mould. Glass U.S. Pat. No. (3,732,053) teaches the use of a pallet, with a tie in place, being inverted and held against downward movement while pressure is exerted through an opening in the pallet to eject the tie from the pallet. However the mould is rotated by being coupled to a rotary head in turn connected to a motor, but that arrangement is complex when large numbers of moulds are to be rotated. Dowsett (U.S. Pat. No. 2,397,728) teaches the aligning of a series of open ended moulds about tensioning wires and fitting end plates between the aligned moulds and about the wires. After the concrete has been cast and hardened in the moulds and about the wires, the end plates are removed to facilitate cutting of wires by torches. Stockmar (U.S. Pat. No. 2,596,052) teaches inversion of moulds to assist in release of the products therefrom, wherein the moulds are positioned against conveyor rollers and are then tilted along with the conveyor rollers to avoid any danger of the cast concrete products tumbling out of the moulds. As far as is known, the above is an accurate summary of the most relevant prior art.
Notwithstanding the above developments however, the method most commonly used in practice is to arrange the multi-cavity moulds on a base frame, pour the concrete, sever the wires between the moulds, and remove the products from the moulds. Most of the operations are relatively simple, but several difficulties are encountered. Firstly if the moulds are of the type having closed ends, the adhesion between a product and the walls of the moulds becomes very great unless there is a large "draw angle," and this is not permitted in most instances with concrete ties. Secondly, some hoisting arrangement is usually employed and this usually requires the use of overhead tracks. Thirdly, the reinforcing wires are frequently positioned by means of interengaging plates or plates having apertures therein, and these are slow to assemble and difficult to release. However although the abovementioned difficulties add considerably to the expense, the most series difficulty of all is that of discharging the cast ties from the moulds having regard to the fact that the moulds move longitudinally.
If the number of ties to be cast in one run of a hopper is considerable (say for example 500), and the number of cavities in a multi-cavity mould is relatively small (say for example 5) then there is a large number of spaces between adjacent moulds, (in this example 99 spaces). Before the spacer bars can be repositioned, these spaces must be enlarged for insertion of spacer bars between the mould ends, this being achieved by moving the moulds along the base frame. The spaces contract upon recovery of the tensioned wires when tension is released after the concrete has set and the spacers removed. These two movements combine, and are additive over the bed length, so that the movement of the furthermost mould is likely to exceed the distance between supports for a continuous hinge, if used. Furthermore, a fixed hinge bar extending along one side of the moulds interferes with spacer bar withdrawal from that side, and it is often inconvenient to withdraw from the other side. It is believed that this is the reason that commercially, most moulded ties are hoisted from the mould cavities and transported away from the moulds by an overhead hoist. The excessive longitudinal movement makes it difficult to achieve an inversion of the moulds owing to interference by the hinge bar supports, and an object of this invention is to provide means whereby the moulds may be readily inverted about longitudinal axes even though they need to move along the base frame.